Coating cellophane and the like



United States Patent Qiifice 2,699,406 Patented Jan. 11, 1955 COATING CELLOPHANE AND THE LIKE Charles M. Rosser, Fredericksburg, Va., assignor to American Viscose Corporation, Philadelphia, Pa., a corporation of Delaware No Drawing. Application November 3, 1951, Serial No. 254,795

13 Claims. (Cl. 117--63) This invention is related to the coating of cellophane and other hydrophilic pellicles with resinous materials and is particularly concerned with a coating procedure in which thermosetting resin precondensates and an acid catalyst are applied to the cellophane film or the like. Such coated films have been used heretofore as wrapping sheets or have been treated with additional coatings, such as moistureproof and/ or heat-sealing coatings, to provide other types of wrapping materials.

The resin pro condensates with which the invention is concerned are those of melamine, urea, or other amines, with an aldehyde, such as acetaldehyde and especially formaldehyde. Various acids have been suggested for use with such resins for the purpose of catalyzing or accelerating the polymerization to the infusible state and also for the purpose of dispersing the precondensate in water to provide a stable aqueous medium adapted to be applied as a coating medium. Among the acids, the most common suggested has been hydrochloric acid because of its inexpensiveness and availability. In coating cellophane with such resin precondensates, the practice heretofore suggested was to follow the impregnation with the aqueous acidic resin dispersion by a step of washing for the purpose of reducing the acid content in order to prevent excessive tendering and weakening of the film as a result of the content of acid incorporated by this coating procedure. However, the Washing out of the acid is accompanied by a washing out of a part of the resin precondensate. It also requires an extensive travel of the film through the washing bath and, of course, the length of such bath is increased in proportion to increases in the speed of travel. Such organic acids as hydroxyacetic and lactic acid have also been used without being subsequently washed out. These acids, when left in the final product, have a plasticizing action thereon but still have the disadvantage of tendering and weakening the film as it ages.

It is an object of the present invention to eliminate the necessity to wash out the acid used in dispersing the resin while avoiding the tenderizing and weakening of the coated film. A further object is to eliminate the need for equipment or vessels for applying a washing liquid. Further objects and advantages of the invention will be apparent from the description thereof hereinbelow.

In general, the invention comprises the impregnation of a cellophane sheet with a dispersion of a thermosetting resin precondensate formed in an aqueous medium by gluconic acid and then, without washing, passing the impregnated sheet into a plasticizing bath which may or may not contain a small amount of an alkaline compound, such as sodium, potassium, ammonium or lithium carbonates, bicarbonates, or hydroxides for neutralizing the acid. Subsequently, the plasticized film may be dried and, if desired, coated with a moistureproof and/ or heatsealing composition of any type.

It has been found that gluconic acid, and especially d-gluconic acid, not only acts as a plasticizer, but it is also free of any deteriorating effect on the cellulosic pellicle. Thus, it may be left on or in the pellicle and stored or put into use over long periods of time under conditions of temperature and humidity normally prevailing without suffering any measurable loss in tensile strength. In making up the precondensate dispersion, the acid itself may be used, or instead, glucano delta lactone may be the starting material. The lactone hydrolyzes to produce the same equilibrium mixture as is present in an aqueous solution of gluconic acid.

The salts of gluconic acid produced by the neutralization when it is used generally are plasticizers for the pellicle. The preferred salts are those of lithium as they are outstanding in their plasticizing effect. If desired, the concentration of plasticizer in the plasticizing bath may be somewhat reduced to compensate for the amount of lithium salt plasticizer that is formed by neutralization of the acid carried into the plasticizing bath. It should be understood that the gluconic acid does not have to be neutralized to prevent tendering, as the acids heretofore used, but for certain purposes a non-acidic pellicle may be desirable, as in the Wrapping of certain foodstuffs.

While reference hereinabove has been made to cellophane sheets, by which is meant sheets: or films of regenerated cellulose, the invention is applicable to other hydrophilic pellicular material, such as sheets of cellulose ethers, especially the alkali-soluble, water-insoluble cellulose ethers formed by the reaction of cellulose with ethylene oxide, methyl chloride, or ethyl chloride. The regenerated cellulose may be produced by the viscose process, the cuprammonium process, or by the denitration of nitrocellulose. The impregnation of the pellicle may be effected prior to its first drying during initial formation of the sheet or film, or it may be: effected upon a dried sheet. Preferably, the impregnation is carried out during the initial production of the sheet while it is still in the Wet gel stage. The impregnation in this stage may be preceded by the removal of excess liquid just before the sheet or film reaches the place where impregnation by the resin precondensate is to be effected. If desired, a portion of the absorbed moisture may be removed prior to impregnation but preferably the moisture content of the sheet or film at the time of impregnation by the resin precondensate is at least 25 to 65% by weight of the wet gel sheet.

The dispersion of resin precondensate may be any of those disclosed in U. S. Patent 2,345,543, particularly of the melamine-formaldehyde variety, gluconic acid being substituted for the acids therein. It may be a urea-formaldehyde precondensate, or it may be a mixed melamine and urea-formaldehyde precondensate, .as well as the urea-containing melamine-formaldehyde precondensate disclosed in U. S. Patent 2,5 64,925, again substituting gluconic acid for the acids of the patent.

When an alkaline compound is used for neutralization, it may be provided in a separate bath through which the impregnated sheet or film is passed prior to passage through the plasticizing bath; however, it is preferable that the neutralizing and plasticizing bath be combined into a single container. 1

The plasticizer bath may contain glycerol, or any other known plasticizer for cellulosic materials provided they do not react with the alkaline lithium compound or its salt to provide an insoluble residue or material. Examples of other plasticizers are sorbitol, ethylene glycol, ethanolamine lactate, ammonium sulfamate, sodium lactate, ethylene oxide condensation products of sorbitol, glycerol or urea, or the like.

The resin precondensate dispersion may contain from 0.5% to 20% by weight, and preferably from 2 to 5% by Weight, of the precondensate. The plasticizing bath may contain from 3 to 10% of the plasticizer, preferably 4 to 5% by weight thereof. In addition, it may be provided with 0.01 to 0.1% of an alkaline compound, preferably just sufficient to be slightly in excess of the acid entering on the film. If a separate neutralizing bath is provided before the plasticizing bath, it may contain 0.01 to 0.1% by weight of the alkaline compound dissolved in water. In addition, a small amount of a fungicide may be incorporated in the neutralizing and/or plasticizing baths.

In the following examples,

the parts and percentages given are by we1ght:

Example I A mixture of 20 parts of partially polymerized, hydrophobic melamineormaldehyde resin, 35 parts urea, 35 parts Water and 8.5 parts of gluconic acid was stirred together at room temperature until the solution was sub- 3v stantially complete. Thissolution was diluted to a 5% concentration. A wet gel regenerated cellulose pellicle was passed through the solution through such a path that all parts of the film wasin contact withthe solution for a period of fifteen seconds; The impregnated film was then passed into an aqueous: bath. containing 5% glycerine. Excess liquid was removed from the film after it left each bath and the plasticized-filmwas then dried.

Example 11- Three mols of melamine were mixed with mols of a 30% aqueous formaldehyde to give a solution having a pH of 9.0. The solution was heated for 30 minutes, cooled, and the resulting; crystalline condensate separated and dried. Fifty partsby weight of the condensate were dissolvedin an aqueous solution containing 25 parts of gluconic acid and -12 '5 parts-of water thus giving a solution containing 25% of the resin. This solution was diluted to 14% resin and} allowed to age at room temperature for 24 hours,until abluish haze developed. A sheet of regenerated cellulose in the wet gel state was then passed through the resin-solution, the immersion taking 20 seconds. After removal of excess impregnant from the film, the fil'm was passed into a plasticizingbath containing 5% glycerineand 0.01% of lithium hydroxide. After removal of'excess plastizing bath, the film was dried. V

Example 111 A wet gel regenerated cellulosefilm was passed continuously during its production and after complete regeneration but before its first drying through an aqueous,

solution containing 10% of a water-soluble'urea-formaldehyde prccondensateand Ai%--of gluconic acid. The passage-through-the bath tools-15 seconds. After removal ofexcess liquid from the im'pregnated film, it proceeded continuously into a:-bathcontaining 0.01% sodium carbonate and 5 g'ly'ce'rine. Afterremoval of excess'moisture, the film wa's-driedc Eic'ample' IV Each ofthe coatedfilmsprodu'ced in the preceding Examples 1 to IIIwere coated with a moistureproof heatscalable lacquer havinglthe" following composition-:-

' Parts Nitrocellulose (11.6% N) 50 Dicyclohexyl phthalate 30 Dibutyl phthalate 10 Dewaxed dammar 7.5 Paraflin wax (M. P. 60 C.) 2.6

After drying of the moistureproof and heat-sealing coating on the films, it was found that they were well anchored so that they resistedse'parat-ion when exposed-to water and that the coatedsheets retained their strength over long periods of time.

It is to be understoodthat-changesand-variations may be made without departing' fro'mthe spirit and scope'ot' the invention as defined in the appended claims:

I claim:

1. A process of treatinga-non-fibrous hydr ophilic cel lulosic pellicle which comprises impregnatingthe-pellicle with a thermosetting resin-precondensate selected from the group consisting of precondensates of formaldehyde with melamine, urea, and mixtures thereof in the-pres- 4|. ence of a small amount of gluconic acid, and subsequently drying the impregnated pellicle.

2. A process according to claim 1 including the step of treating the pellicle with a plasticizer therefor prior to drying.

3. A process as defined in claim 2 in which naturalization of acid carried by the impregnated pellicle is efiected by passing the pellicle through an aqueous bath containing a plasticizerfor the pellicle and a small proportion of analkaline-compound of the alkali metal group.-

4. A process as defined in claim-3 in whichthe-pellicle is a regenerated cellulose pellicle.

5. A process as defined in claim 4 in whichthe'resin precondensate is on'eof melamine and formaldehyde.

6. A process as defined in claim 5 in which the impregnation is effected'durin'g the manufacture of the regenerated cellulose pellicle in continuous fashion after the complete regeneration of the pellicle but prior to s'u'fficie'nt drying thereotto terminate th'ew'et' gel r idit i bh of the pellicle.

7. A process as defined in claim 6ir1'"\vhi'ch"the"c6ii centration of the resin precondensate in'solu'tion is' b'e tween /2 and 20% by Weight and the concentration of the p'lasticizer is between"?! and 10% by weight.

8. A process of treating a non-fibrous hydrophilic eel lulosic pellicle which comprises impregnating the pellicle with a theroms'ettin'g resin precondensate of melamine and formaldehyde in the presence of a small amount of gluconic acid, and subsequently drying the impregnated pellicle.

9. A process of'treating a non-fibrous hy'dr'o'philic' cel lulosic pellicle which comprises impregnatingthe'pellicle with a thermosettin-g'resin'precondensate of ureaand' formaldehyde in the presence of a smart amount of; gluconic acid, and subsequently drying the impregnated pellicle.

10. A process of treating a non fibrous hydrophiliccl lulosic' pellicle which comprises impregnating the 'pelli "e with a thermosetting resin prec'o'ri'dens'ate' of melamine and'formaldehyd'e in the presence of a small'amou'ntof gluconic acid, plasticizing and neutralizing the pellicle by passing it through an aqueous bath containing aplas ticizer for the pellicle and a small proportion of'an alka line compound of the alkali metal group:

11. A process as defined in claim l0'inwh'ich the "pelhole" is of regenerated cellulose.

12. Aprocess as defined in claim 11 in'wh'ich thealkaline compound is lithium hydroxide.

13. A process of treating a non-fibrous hydrophilic' cellulosic pellicle which comprises subjecting the pellicle to an aqueous solution containing /z" to 20% by weight of a thermosetting resin precondensate" of-rriel'amifie and formaldehyde, 3 to 10% by weight of glycerol; ari'd'a' small amount on the order of 4% of glueofii'c'acid arid subsequently drying the impregnated pellicle.

References Cited in' the file of this patent UNITED STATES PATENTS 2,095,129; Dr'ew Oct." 5,193? 2,197,357 Widrner, et'al. Apr. 16,1940 2,280,829 Iebens Apr. 28; 1942 2,417,014 Pollard Mar'. 4'; 1947 2,575,443 Cornwell Nov; 20, 1951 

10. A PROCESS OF TREATING A NON-FIBROUS HYDROPHILIC CELLULOSIC PELLICLE WHICH COMPRISES IMPREGNATING THE PELLICLE WITH A THERMOSETTING RESIN PRECONDENSATE OF MELAMINE AND FORMALDEHYDE IN THE PRESENCE OF A SMALL AMOUNT OF GLUCONIC ACID, PLASTICIZING AND NEUTRALIZING THE PELLICLE BY PASSING IT THROUGH AN AQUEOUS BATH CONTAINING A PLASTICIZER FOR THE PELLICLE AND A SMALL PROPORTION OF AN ALKALINE COMPOUND OF THE ALKALI METAL GROUP.
 11. A PROCESS AS DEFINED IN CLAIM 10 IN WHICH THE PELLICLE IS OF REGENERATED CELLULOSE. 